Electrodeposition and characterization of flower-like Na(VO)2(PO4)2.4H2O: structure, electronic and optical properties

Vanadium phosphate-based materials are widely studied as cathodes for sodium-ion batteries due to their high theoretical capacity, good structural stability and rich redox chemistry involving vanadium oxidation states from V 2 + to V 5+ . In this work, the sodium vanadyl phosphate hydrate, Na(VO) 2 (PO 4 ) 2 .4H 2 O, was synthesised for the first time by chronoamperometric electrodeposition from an aqueous VOSO 4 /H 3 PO 4 solution (pH = 2) in the presence of 0.01 M NaNO 3 . X-ray diffraction (Rietveld refinement) highlighted a well-crystallized triclinic phase with an average crystallite size of around 41.1 nm. FTIR spectroscopy confirmed the presence of structural water and characteristic V-O and P-O vibrations, while SEM/EDS analysis showed a nanoplatelet-like morphology aggregated into flower-like structures. Thermogravimetric analysis indicted a mass loss corresponding to the removal of four water molecules, yielding the anhydrous phase with stability up to 500°C. First-principles (GGA+ U ) calculations revealed a semiconducting behaviour with a band gap near to 1.46 eV and notable UV absorption. Overall, the results demonstrate electrodeposited Na(VO) 2 (PO 4 ) 2 .4H 2 O is a structurally robust and promising candidate for sodium-ion battery applications.